fix(log): mkdir parent arg should be parents (#464)

Author: juliekoubova

lua/plenary/log.lua

@@ -137,7 +137,7 @@ log.new = function(config, standalone)
     if config.use_file then
       local outfile_parent_path = require("plenary.path"):new(outfile):parent()
       if not outfile_parent_path:exists() then
-        outfile_parent_path:mkdir { parent = true }
+        outfile_parent_path:mkdir { parents = true }
       end
       local fp = assert(io.open(outfile, "a"))
       local str = string.format("[%-6s%s] %s: %s\n", nameupper, os.date(), lineinfo, msg)